The present invention concerns a photovoltaic cell, also called a solar cell, which can be used to form a dial for a watch or another electronic apparatus and to provide the watch or apparatus with electric power.
The invention concerns more particularly a colored photovoltaic cell of the type including from the bottom to the top a substrate, a bottom reflecting electrode placed on the substrate or integrated therein, an active photodiode part formed of semiconductor layers, and a top electrode. The invention also concerns a watch dial including such a colored photovoltaic cell.
The use of photovoltaic cells as dials for wristwatches encounters several problems which may be of a technical or aesthetic nature. First, when the watch is worn, the cell or a set of cells connected in series has to generate sufficient power not only in the short term, but also in terms of the annual energy balance of the storage element (accumulator), to take account of periods of poor light, in particular the winter. As the surface area of a watch dial is limited, the cell has to have sufficient photovoltaic efficiency. Further, one wishes to have as thin a cell as possible. Photovoltaic cells made of amorphous silicon are advantageous in this respect, particularly if the substrate is metal in nature and forms one of the current collector electrodes.
On the other hand, aesthetic criteria are important in the field of horology. They result in the use of a single cell being preferred to that of several juxtaposed cells connected in series, in order to avoid the lines of separation and interconnection which remain visible between the cells. One primordial aesthetic aspect is that of color. The amorphous silicon cells deposited on a metal substrate have in themselves a greyish appearance which is unattractive. Various publications proposing solutions to avoid this drawback will be mentioned hereinafter. Finally, in addition to obtaining attractive, varied and sufficiently light colors, one may wish the dial to give either specular reflection, or diffuse reflection.
European Patent No. 872 783 discloses a watch dial, formed of a single hydrogenated amorphous silicon photovoltaic cell (xcex1-Si:H), whose top electrode is formed of a transflective, i.e. semi-reflecting, metal layer, which preferably reflects between 60% and 85% of the incident light. This permits a simple and thin construction, giving the dial a metallic lustre, but the photovoltaic efficiency is reduced by the fact that most of the incident light is reflected towards the exterior or absorbed in the metal layer and thus does not reach the silicon.
Another category of solutions known, in particular, from European Patent Nos. 788 037 and 819 995, consists in placing on the photovoltaic cell a plate formed of various translucent colored layers themselves providing the desired appearance of the dial. However, these plates have to be diffusing to conceal the silicon, which on the one hand reduces the quantity of light reaching the photodiode and on the other hand gives the dial a milky appearance lacking in lustre. Moreover, this plate increases the total thickness of the construction.
The object of the present invention is to create a photovoltaic cell allowing the aforementioned drawbacks to be avoided when it is used as a watch dial or an apparatus imposing the same aesthetic criteria. In particular, the invention should allow a wide choice of colors for the dial, without excessively reducing the portion of incident light which reaches the photodiode. An additional object is to offer the designer the choice between a specular reflection and a diffuse reflection of the incident light.
According to a first aspect of the invention, there is provided a photovoltaic cell of the type indicated hereinabove, characterised in that the pair of respective thicknesses of the top electrode and the active photodiode part is selected as a function of the respective refractive indices of their materials so as to generate an interferential reflection of the incident light according to a predetermined reflection spectrum.
Thus, the multi-layered structure of the photovoltaic cell constitutes a reflective interferential filter with which it is possible to obtain a wide range of colors of the reflected light, simply by an appropriate choice of the thicknesses of the layers generating the interferential reflection, by using the constituent materials of the photovoltaic cell. In practice, said thicknesses are selected from the ranges of thickness compatible with the proper operation of the photovoltaic cell, in particular with regard to the power which it provides.
The parts of the spectrum in which a substantial fraction of the incident light is reflected towards the exterior may be relatively narrow, so that most of the incident ambient light is absorbed by the photodiode to generate the electric power. All other conditions being equal, this type of cell thus allows currents to be obtained which are significantly higher than those of a cell with a semi-reflective metallic top electrode covered with colored gloss. The photodiode can advantageously be made in a conventional manner from hydrogenated amorphous silicon. Preferably, the substrate is metal and serves both as the bottom electrode and the reflector.
Preferably, the active part of the silicon photodiode has a thickness comprised between 100 and 600 nm and the top electrode has a thickness comprised between 60 and 300 nm, the pairing of these two thicknesses resulting in a determined color of the reflected light. In particularly preferred ranges of thickness, the active part of the silicon photodiode has a thickness comprised between 250 and 450 nm and the top electrode has a thickness comprised between 70 and 150 nm. Below these lower limits, the sheet resistance of the top electrode increases, creating undesirable ohmic losses, and the power conversion in the silicon is reduced if the thickness of the silicon is too small. Above the aforementioned upper limits, the deposition of the top electrode becomes too expensive and the mechanical hold of the silicon on the substrate becomes problematic.
In order to further enlarge the palette of available colors, a development of the invention consists in that the top electrode is covered with a clear or transparent lacquer, which may also be diffusing. This layer may further contain colorants or pigments, thus exhibiting a predetermined absorption spectrum in order to modulate the color of the reflected light by absorbing certain wavelengths. These techniques allow dials simulating the appearance and colors of conventional watch dials to be obtained.